Head smut is a soil-borne and systemic disease in maize (Frederiksen 1977) caused by the host-specific fungus Sphacelotheca reiliana (Kühn) Clint. The teliospores from sori buried in soil are the primary source of infection, and can survive three years in soil without loss of any infection capacity (Wu et al. 1981). The fungus infects seedlings through roots or coleoptiles during and after seed emergence (Krüger 1962). In an infection of a susceptible variety the plants continue normal vegetative growth, but some may be stunted (Matyac and Kommedahl 1985a). At maturity sori replace ears or tassels of the infected plants, resulting in nearly no maize yield for the plant. The proportion of infected plants in an infected field could amount to 80% (Frederiksen 1977). Jin (2000) reported the incidence of this disease varied from 7.0% to 35.0%, some even reaching 62.0%, resulting from the cultivation of susceptible cultivars. In Northern China, head smut causes yield loss of up to 0.3 million tons annually (Bai et al. 1994). It was reported that maize in Southern Europe, North America, and Asia also seriously suffer from this disease (Xu et al. 1999). Considering both economic and ecological elements, cultivation of resistant varieties is an effective way to control epidemics of head smut. Breeding for multiple resistant genes/QTLs against head smut into elite maize varieties would be a promising way to improve the resistance against this disease.
To date, many researches have studied genetic models conferring resistance against head smut. Mei et al. (1982) reported that resistance against head smut was controlled by partially dominant nuclear genes with no difference being found in reciprocal crosses. Ma et al. (1983) reported maize resistance to head smut was a quantitative trait, affected by partial resistance genes and their non-allelic interactions. Stromberg et al. (1984) discovered that F1 population showed an intermediate disease incidence between resistant and susceptible parents. Ali and Baggett (1990) reported additive and dominant genetic actions were preponderant under different treatments. Bernardo et al. (1992) studied genetic effect of resistance gene(s) by using generation mean analysis, suggesting that additive effect is decisive, while the dominant and epistatic effects are weak. Shi et al. (2005) reported that apart from additive effect, over-dominance also plays a key role in resistance against head smut. It is obvious that resistance against head smut in maize may involve in a number of genetic elements and act in a complex way.